In general, substrates used to manufacture flexible wiring substrates are roughly divided into adhesive copper clad laminates in which a copper foil serving as a conductor layer is bonded together on an insulating film by using an adhesive (see, e.g., Patent Document 1), and adhesiveless copper clad laminates in which a copper film layer serving as a conductor layer is directly formed on the insulating film by a dry plating method or a wet plating method without using an adhesive in between.
Here, when using the adhesive copper clad laminates, adhesive flexible wiring substrate can be manufactured by forming a desired wiring pattern on a substrate by a subtractive method and, when using the adhesiveless copper clad laminates, an adhesiveless flexible wiring substrate can be manufactured by forming a desired wiring pattern on a substrate by a subtractive method or an additive method, but use of the adhesive copper clad laminates that can be manufactured by a simple manufacturing method at low cost forms a mainstream in a conventional technology.
Meanwhile, with recent density growth of electronic devices, a wiring substrate whose wiring width pitch is also narrowed has been demanded.
However, in manufacture of adhesive copper clad laminates, when a wiring portion is formed on a copper film layer provided on an insulating film as a substrate by etching in accordance with a desired wiring pattern to manufacture a wiring substrate, so-called side etching that a side surface of the wiring portion is etched occurs, and hence a cross-sectional shape of the wiring portion tends to have a trapezoidal shape spreading toward the bottom.
Therefore, when etching is carried out till electrical insulating properties are assured between wiring portions, a wiring pitch width becomes too wide, and hence there is a limit in narrowing a pitch of the wiring portion on a wiring substrate as long as adhesive copper clad laminates in which a generally conventionally used copper foil having a thickness of 35 μm is bonded to an insulating film through an adhesive is utilized.
Therefore, a thin copper foil bonded substrate having a thickness not greater than 18 μm is used in place of the conventional copper foil bonded substrate having a thickness of 35 μm so that a width of a shape spreading toward the bottom obtained by side etching is reduced to narrow a pitch of the wiring portion on the wiring substrate.
However, since such a thin-walled copper foil has small rigidity and poor handling properties, there is adopted a method of temporarily bonding a reinforcing material such as an aluminum carrier to the copper foil to increase the rigidity, then bonding the copper foil to the insulating film, and further removing the aluminum carrier, but this method takes trouble and time and has a problem of poor operability and productivity.
Further, such a thin copper foil has a problem in a manufacturing technology, e.g., an increase in film defects due to unevenness in film thickness or occurrence of pin holes or cracks, the copper foil itself becomes difficult to be manufactured as a thickness of the copper foil is reduced, and a manufacturing price is increased, thereby losing cost merits of the adhesive flexible wiring substrate.
In particular, a demand for a wiring substrate having a wiring portion with a narrow width and a narrow pitch that cannot be manufactured unless a copper foil having a thickness of ten-odd μm or below or approximately several μm is used has been recently increased, and a wiring substrate using adhesive copper clad laminates has the above-explained technical problem as well as a manufacturing cost problem.
Thus, attention is paid to a double layer flexible wiring substrate using adhesiveless copper clad laminates in which a copper film layer can be directly formed on an insulating film without utilizing an adhesive in between.
According to such adhesiveless copper clad laminates, a copper conductor layer is directly formed on an insulating film without using an adhesive, a thickness of the board itself can be thereby reduced, and there is an advantage that a thickness of the copper conductor film to be applied can be adjusted to an arbitrary thickness.
Furthermore, when manufacturing such adhesiveless copper clad laminates, an electrolytic copper plating method is usually adopted as means for forming a copper conductor layer having a uniform thickness on an insulating film, but it is general to form a base metal layer on the insulating film to which the electrolytic copper plating film is applied to provide electroconductivity on the entire surface and then perform electrolytic copper plating processing (see, e.g., Patent Document 2).
Meanwhile, although it is known to use a dry plating method, e.g., a vacuum deposition method or an ion plating method, to obtain the base metal layer on the insulating film, many pin holes having a size of several-ten μm to several-hundred μm are produced in the film layer obtained by such a dry plating method, and hence an insulating film exposed portion due to the pin holes are often generated in the base metal layer.
In a conventional technology, generally, it is said that a range of 35 μm to 50 μm is appropriate as a thickness of a copper electroconductive film required for a wiring line in this type of flexible wiring substrate, but a width of the wiring line is also approximately several-hundred μm, a defect in the wiring portion due to presence of pin holes having a size of several-ten μm rarely occurs.
However, when obtaining a flexible wiring substrate having a wiring portion with a narrow width and a narrow pitch intended by the present invention, it is desirable to set a thickness of a copper film required to form a wiring portion to a very small thickness that is not greater than 18 μm or, preferably, not greater than 8 μm or, ideally, approximately 5 μm as described above, and a possibility of occurrence of a defect in the wiring portion is increased.
Explaining this situation while taking manufacture of a flexible wiring substrate by, e.g., a subtractive method using adhesiveless copper clad laminates in which a copper film layer having a desired thickness is formed on an insulating film having a base metal layer as an example, formation of a wiring portion pattern is carried out at the following steps.
(1) A resist layer having a desired wiring portion pattern by which a wiring portion alone is masked and a copper conductor layer of a non-wiring portion is exposed is provided on the copper conductor layer; (2) the exposed copper conductor layer is removed by chemical etching processing; and (3) the resist layer is peeled and removed at last.
Therefore, in case of using a substrate on which a copper film layer having a very small thickness, e.g., 5 μm is formed to manufacture a wiring substrate having a narrow wiring width, e.g., 15 μm and a narrow wiring pitch, e.g., 30 μm, a size of bulky ones of pin holes produced in a base metal layer of the substrate by the dry plating processing reaches an order of several-ten μm to several-hundred μm, and hence an insulating film exposed part due to the pin holes cannot be sufficiently filled when an electrolytic copper plating film having a thickness of approximately 5 μm is formed, whereby this exposed part, i.e., a defective part of the conductor layer reaches the wiring portion and the wiring portion gets chipped at positions of the pin holes to become a wiring defect, or even if not so, an adhesion failure of the wiring portion is led.
As a method of solving the above-described problem, a method of forming a base metal layer on an insulating film by a dry plating method and then applying a copper film layer as an intermediate metal layer obtained by electroless plating to coat an exposed part of the insulating film due to each pin hole has been proposed (see, e.g., Patent Document 3).
However, according to this method, an exposed part of the insulating film due to a pin hole can be assuredly eliminated to some extent but, on the other hand, it is known that a plating liquid, its preprocessing liquid, or the like used in electroless copper plating processing enters a space between the insulting film and the base metal layer from already formed large and small various pin hole parts, and this may possibly becomes a factor that obstructs adhesion properties of the base material layer and adhesion properties of a conductor layer subsequently formed by electrolytic copper plating, and hence this method is not a sufficient countermeasure.
Further, for example, Patent Document 4 proposes a non-adhesive flexible laminate including a polymer film having a plasma-processed surface, a nickel tie coating layer containing nickel or a nickel alloy that has adhered to the plasma-processed surface, a copper coating layer that has adhered to the nickel layer, and another copper layer that has adhered to the copper coating layer, and discloses the nickel tie coating layer whose metal for a nickel alloy is selected from a group including Cu, Cr, Fe, V, Ti, Al, Si, Pd, Ta, W, Zn, In, Sn, Mn, Co, and two or more mixtures of these metals. Specifically, as useful Ni alloys, there are Monel (approximately 67% Ni, and 30% Cu), Inconel (Approximately 76% Ni, 16% Cr, and 8% Fe) and others. This document explains that the obtained laminated film is superior in initial peel strength, peel strength after solder floating, and peel strength after a heat cycle, but does not describe about excellence in properties of a composite metal film.
Furthermore, for example, Patent Document 5 discloses that a first thin layer formed of at least one type of metal selected from a group including nickel, chrome, molybdenum, tungsten, vanadium, titanium, and manganese is formed on a polyimide side by a vacuum film forming method, a second thin layer with a predetermined thickness made of copper is formed thereon by the vacuum film forming method, and a third thin layer with a predetermined thickness made of copper is formed on the second thin layer by electroplating with a predetermined current density in order to improve heat-resisting adhesiveness of a polyamide/metal interface on a polyimide side of a copper-clad polyimide film provided by applying and hardening a polyimide varnish on a copper foil and improve productivity of this composite base material and durability and reliability of a final electrical product, but chrome alone is described as the first thin layer in an embodiment thereof, and excellence in properties of a composite metal film is not explained.
Likewise, for example, Patent Document 6 also discloses provision of a flexible printed wiring substrate by superimposing on one side or both sides of a plastic film a laminated body constituted of an evaporated layer of nickel, cobalt, chrome, palladium, titanium, zirconium, molybdenum, or tungsten and an electron beam heating evaporated copper layer that is superimposed on the evaporated layer, made of an aggregation of evaporated particles whose diameter falls in a range of 0.007 to 0.850 μm, and has a desired circuit formed thereon, and a mask layer that has no circuit formed thereon and is constituted of a mask layer made of an insulative organic material in order to provide a reliable inexpensive flexible printed wiring substrate superior in interlayer adhesion, heat resistance, chemical resistance, flexibility, and electrical characteristics, but a chrome evaporated layer alone is described in an embodiment of this document, and excellence in characteristics of a composite metal film is not explained at all.    Patent Document 1: Japanese Examined Patent Application Publication No. 1994-132628    Patent Document 2: Japanese Examined Patent Application Publication No. 1996-139448    Patent Document 3: Japanese Examined Patent Application Publication No. 1998-195668    Patent Document 4: PCT National Publication No. 2000-508265    Patent Document 5: Japanese Examined Patent Application Publication No. 1995-197239    Patent Document 6: Japanese Examined Patent Application Publication No. 1993-283848